The present invention is directed to a method of and an apparatus for removing residual plastic from plastic processing equipment. Such equipment can include molds, extruding screws, extruding pipes and the like. The present invention can also be used for removing the residue of materials other than plastics, such as enamel and varnish.
The equipment used in processing plastics has traditionally been cleaned with either heat or chemical solvents. However, heat can degrade the equipment; for example, it can degrade steel tools by altering the grain structure of the steel. Chemical solvents may also degrade certain tools; more importantly, it is often difficult to dispose of the used solvents in an environmentally acceptable manner.
Some industries use impact cleaning methods such as scraping and blasting. For example, in the printing industry, cans having ink residue are frozen, and the frozen ink is scraped off. However, such methods are not suitable in all industries. The scraping or other impact may damage high-precision equipment; also, some tools, such as extruding screws, are too intricate to scrape efficiently.
It is also known in the art to clean equipment with pressurized gas, e.g., CO.sub.2. However, the force of pressurized CO.sub.2 does not suffice to clean the equipment in all cases.
The combination of extreme cold and blasting has been used to deflash molded articles, or in other words, to remove the residual material left on the articles between the interfacing mold surfaces. An example of this technique is taught in U.S. Pat. No. 4,979,338 to Schmitz, II et al. Similar techniques have been employed to remove an adherent coating from an article, as taught in U.S. Pat. No. 4,627,197 to Klee et al and in U.S. Pat. No. 5,761,912 to Popp et al. However, those techniques do not effectively and efficiently remove all of the residue without the need for impact cleaning.